I have done hundreds of mobile radio installations (public safety and ham radio). I have yet to find any vehicle (and that includes former military Humvees and MRAPS) where there was not some place to get through the firewall. Existing feed throughs with grommets or, as a last resort, drill a separate hole. It does not need to be a large hole since you simply need to route the positive wire through it. The negative (ground) can be affixed to the body inside the passenger compartment. If you drill a new hole use a grommet to protect the wire and silicone any space between the wire and grommet to prevent moisture intrusion.